Composition for phosphate film optimizing mn content and a method for phosphate treatment of zn electric-plated steel sheet

ABSTRACT

A composition for a phosphate film of a Zn electric-plated steel sheet may comprise zinc (Zn), nickel (Ni), and manganese (Mn), wherein a content of Mn is 6 to 8 wt %.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims priority to Korean Patent Application No. 10-2016-0071139, filed on Jun. 8, 2016, the entire contents of which is incorporated herein for all purposes by this reference.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a composition for a phosphate film with an optimized content of manganese (Mn) and relates to a method for phosphate treatment of a zinc (Zn) electric-plated steel sheet, and more particularly, to a composition for a phosphate film of a Zn electric-plated steel sheet and a method for phosphate treatment of the Zn electric-plated steel sheet which can solve a partial dissolution problem of the phosphate film of a Zn plated steel sheet for a vehicle by optimizing a Mn content, a molecular size of the phosphate, and a coating weight of the phosphate film, as a composition for the phosphate film containing Zn, nickel (Ni), and Mn.

Description of Related Art

In general, when a Zn electric-plated steel sheet used as a vehicle hood steel sheet is molded, a plating chip is generated by low hardness. The generated plating chip adheres to the mold to cause an unevenness defect on the surface of the steel sheet.

In order to prevent the problem caused by the low hardness, when the Zn electric-plated steel sheet is used for a vehicle body, generally, the Zn electric-plated steel sheet is applied by adding a post-treatment step. A phosphate film is applied during the post-treatment step, and the phosphate film is partially dissolved by strong alkalis added in a pre-treatment process thereby causing problems such as stripes generated after electrodeposited coating.

That is, in the related art, for enhancing moldability and weldability, a phosphate film added with Zn and Ni is used for post-treatment, but there is a problem in that plating is peeled off during molding due to a lack of wear resistance. Further, there is a problem in that the phosphate film is partially dissolved by the pretreatment coating and thus an additional phosphate process needs to be performed.

As a result, the present invention is directed to a composition for a phosphate film and a method for phosphate treatment of a Zn electric-plated steel sheet which prevents a partial dissolution problem of the phosphate film generated by the pre-treatment coating of the Zn electric-plated steel sheet.

The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.

BRIEF SUMMARY

Various aspects of the present invention are directed to providing a composition for a phosphate film and a method for phosphate treatment of a Zn electric-plated steel sheet with an advantage of preventing a partial dissolution problem of the phosphate film generated in pre-treatment of the Zn electric-plated steel sheet by optimizing a Mn content of the composition for the phosphate film, a molecular size of the phosphate, and a coating weight of the phosphate film.

Various aspects of the present invention are directed to providing a composition for a phosphate film and a method for phosphate treatment of a Zn electric-plated steel sheet with advantages of preventing a pinhole defect by enhancing moldability, refining and optimizing the molecular size of the phosphate, having a beautiful appearance and improving marketability after electrodeposited coating by implementing a uniform state of a film surface as compared with the related art.

Various aspects of the present invention are not limited to the aforementioned disclosure. The foregoing descriptions of specific aspects and the following exemplary embodiments of the present invention were chosen and described in order to explain certain principles of the invention and their practical application, to thereby enable those skilled in the art to make and utilize various exemplary embodiments of the present invention, as well as various alternatives and modifications thereof.

An exemplary embodiment of the present invention provides a composition for a phosphate film of a Zn electric-plated steel sheet containing Zn, Ni and Mn, in which the Mn content is 6 to 8 wt %.

A molecular size of the phosphate may be 1.5 to 2.5 μm.

A coating weight of the phosphate film coated on the Zn electric-plated steel sheet may be 1.6 to 2.0 g/m².

The composition for the phosphate film may contain Zn₂Mn(PO₄)₂4H₂O and Zn₂Fe(PO₄)₂4H₂O.

Another exemplary embodiment of the present invention provides a Zn electric-plated steel sheet coated with the composition for the phosphate film.

Another exemplary embodiment of the present invention provides a method for coating a phosphate film of the Zn electric-plated steel sheet, comprising steps: forming the phosphate film on the Zn electric-plated steel sheet by treating the composition for the phosphate film; and washing the Zn electric-plated steel sheet with the phosphate film.

The method for coating the phosphate film on the Zn electric-plated steel sheet may further include degreasing, surface-modifying, and washing the Zn electric-plated steel sheet.

A surface modifier for modifying the surface of the Zn electric-plated steel sheet may be a Zn surface modifier and the concentration thereof is 2.0 to 3.5%.

According to the exemplary embodiments of the present invention, in the composition for the phosphate film of the Zn electric-plated steel sheet and the method for phosphate treatment of the Zn electric-plated steel sheet, even at pH 12 or higher, it is possible to prevent a partial dissolution problem of the phosphate film of the Zn electric-plated steel sheet by obtaining low alkali solubility.

According to the composition for the phosphate film of the Zn electric-plated steel sheet and the method for phosphate treatment of the Zn electric-plated steel sheet of the present invention, it is possible to enhance the moldability of the Zn electric-plated steel sheet, prevent a pinhole defect by refining and optimizing the molecular size of the phosphate, and obtain a beautiful appearance by implementing a uniform state of the phosphate film, thereby significantly improving the marketability.

The methods and apparatuses of the present invention have other features and advantages which will be apparent from or are set forth in more detail in the accompanying drawings, which are incorporated herein, and the following Detailed Description, which together serve to explain certain principles of the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view illustrating a Zn electric-plated steel sheet in the related art. 10 represents steel sheets. 11 represents a Zn layer. 12 represents a phosphate film comprising Zn and Ni.

FIG. 2 is an enlarged photograph illustrating a Zn electric-plated steel sheet in the related art.

FIG. 3 is an enlarged photograph illustrating a phosphate film after pre-treatment according to Comparative Examples and an Example of the present invention.

FIG. 4A, FIG. 4B, and FIG. 4C are EDX graphs after pre-treatment according to Comparative Examples and an Example of the present invention.

FIG. 5 is a graph illustrating a dissolved amount of phosphate after pre-treatment according to Comparative Examples and an Example of the present invention.

FIG. 6 is an enlarged photograph illustrating a dissolution degree by alkali according to a molecular size of phosphate in Comparative Examples and an Example of the present invention.

FIG. 7 is an enlarged photograph illustrating a surface of a coating film according to Comparative Examples and an Example of the present invention.

FIG. 8 is a graph illustrating solubility according to acidity of a phosphate film in Comparative Examples and an Example of the present invention.

FIG. 9 is a graph illustrating solubility according to acidity of a phosphate film in the related art and an exemplary embodiment of the present invention.

FIG. 10 is a cross-sectional view illustrating a Zn electric-plated steel sheet according to an exemplary embodiment of the present invention. 10 represents steel sheets. 11 represents a Zn layer. 13 represents a phosphate film comprising Zn, Ni, and Mn.

It should be understood that the appended drawings are not necessarily to scale, presenting a somewhat simplified representation of various features illustrative of the basic principles of the invention. The specific design features of the present invention as disclosed herein, including, for example, specific dimensions, orientations, locations, and shapes will be determined in part by the particular intended application and use environment.

In the figures, reference numbers refer to the same or equivalent parts of the present invention throughout the several figures of the drawing.

DETAILED DESCRIPTION

Reference will now be made in detail to various embodiments of the present invention(s), examples of which are illustrated in the accompanying drawings and described below. While the invention(s) will be described in conjunction with exemplary embodiments, it will be understood that the present description is not intended to limit the invention(s) to those exemplary embodiments. On the contrary, the invention(s) is/are intended to cover not only the exemplary embodiments, but also various alternatives, modifications, equivalents and other embodiments, which may be included within the spirit and scope of the invention as defined by the appended claims.

Currently, it has been noted as a problem in that a Zn electric-plated steel sheet which has been used as a vehicle steel sheet has low hardness, and as a result, a post-treatment step for enhancing the hardness is added. FIG. 1 is a cross-sectional view illustrating a Zn electric-plated steel sheet in the related art, and Zn and Ni has been treated on the surfaces of a Zn layer (11) that is electric-plated with Zn on steel sheets (10).

Meanwhile, in the related art, in order to enhance moldability and weldability, a technique of coating a phosphate film containing zinc, nickel, or the like as post-treatment has been pointed out as a problem in that wear resistance is lack. FIG. 2 is an enlarged photograph illustrating a Zn electric-plated steel sheet in the related art, and a phenomenon in which the plated surface of the steel sheet is peeled may be verified.

That is, in the related art, for enhancing moldability and weldability, a phosphate film (12) comprising Zn and Ni is used in the post-treatment step, but there is a problem in that plating is peeled off during molding due to a lack of wear resistance. Further, there is a problem in that the phosphate film is partially dissolved by a coating pre-treatment process and thus an additional phosphate process needs to be performed.

As a result, the present invention is directed to providing a composition for a phosphate film and a method for phosphate treatment of a Zn electric-plated steel sheet which prevents a partial dissolution problem of the phosphate film generated by the pre-treatment of the Zn electric-plated steel sheet.

The composition for the phosphate film of the Zn electric-plated steel sheet of the present invention is a composition for a phosphate film (13) comprising Zn, Ni, and Mn, where the Mn content is 6 to 8 wt %.

The following Table 1 illustrates the Mn content of the phosphate film according to Comparative Examples and an Example of the present invention.

TABLE 1 Example Mn content (wt %) Comparative Example 1 1-3 Comparative Example 2 4-6 Example 1 6-8 Comparative Example 3  8-10

As illustrated in Table 1, in the present invention, a phenomenon in which a phosphate film becomes nonuniform after alkaline degreasing by maintaining the Mn content of the phosphate film to 6 to 8 wt %. Mn may be added to the composition for the phosphate film in a compound form, and the compound reacts with the steel sheet in the phosphate process to be grown on the surface of the steel sheet in a form such as particles.

Meanwhile, in the composition for the phosphate film of the Zn electric-plated steel sheet of the present invention, the Ni content is preferably 0.5 to 1.5 wt %, and Zn and phosphorus occupy almost all of the composition except for manganese and nickel. The Zn content is preferably 55 to 65 wt % and the remaining content is phosphorus.

FIG. 3 is an enlarged photograph illustrating a phosphate film after pre-treatment according to Comparative Examples and Example of the present invention and FIG. 4A, FIG. 4B, and FIG. 4C are EDX graphs after pre-treatment according to Comparative Examples and Example of the present invention.

In Comparative Example 1, it can be seen that the phosphate is excessively dissolved due to the lack of the Mn content in the phosphate film and thus the film is much nonuniformly formed (see FIG. 3). Meanwhile, even in Comparative Example 2, it can be seen that the phosphate is partially dissolved due to the lack of the Mn content of the phosphate film and thus the film is still not uniformly formed (see FIG. 3). On the contrary, in Example 1, it can be seen that since the Mn content of the phosphate film is optimized to 6 to 8 wt %, the phosphate film is uniformly formed (see FIG. 3).

That is, it can be seen that when the Mn content of the phosphate film is low, the phosphate film is dissolved after the alkaline degreasing. In contrast, like Example 1, when the Mn content is 6 to 8 wt %, a uniform surface of the electro-plated steel sheet may be obtained even after the alkaline degreasing. On the contrary, when the Mn content is 8 wt % or more, Mn is added more than necessary in the composition for the phosphate film and thus, there is a problem in that mass productivity may be deteriorated.

FIG. 5 is a graph illustrating a dissolved amount of phosphate after a pre-treatment process according to Comparative Examples and an Example of the present invention. Like Example 1, it can be seen that when the Mn content of the composition for the phosphate film is 6 to 8 wt %, the solubility of the phosphate is 0.05 g/m² or less as the lowest level.

Meanwhile, in the composition for the phosphate film of the Zn electric-plated steel sheet of the present invention, the molecular size of the phosphate is preferably 1.5 to 2.5 μm.

The following Table 2 illustrates molecular sizes of phosphate according to Comparative Examples and an Example of the present invention.

TABLE 2 Example Size of phosphate (μm) Comparative Example 4 1.5 or less Example 2 1.5 to 2.5 Comparative Example 5 2.5 to 5

As illustrated in Table 2, the present invention intends to minimize a pinhole defect in the coating process by optimizing the molecular size of the phosphate in the composition for the phosphate film to 1.5 to 2.5 μm.

FIG. 6 is an enlarged photograph illustrating a dissolution degree by alkali according to a molecular size of phosphate in a Comparative Example and an Example of the present invention.

Like Comparative Example 4, it can be seen that when the molecular size of the phosphate is 1.5 μm or less, the phosphate starts to be partially dissolved from 3 minutes after exposure to alkali, a lot of zinc oxide is formed in advance after 10 minutes, and most of phosphate is lost after 60 minutes (see FIG. 6).

On the contrary, like Example 2, it can be seen that when the molecular size of the phosphate is 1.5 to 2.5 μm, a surface dissolution reaction is almost not shown at 3 minutes after exposure to alkali. Further, it can be seen that zinc oxide starts to be formed after 10 minutes and a small amount of zinc oxide is formed on the surface after 60 minutes (see FIG. 6).

Meanwhile, FIG. 7 is an enlarged photograph illustrating a surface of a coating film according to Comparative Examples and an Example of the present invention.

After anti-corrosion, that is, anti-salt hot water test according to the molecular size of the phosphate, surface states of a complete coating film and an electrodeposited coating film are illustrated (FIG. 7). In the case of the complete coating film, like Comparative Example 5, it can be seen that when the molecular size of the phosphate is 2.5 μm or more, a blister with a maximum length of 2 mm is formed. In the case of the electrodeposited coating film, like Comparative Example 4, it can be seen that when the molecular size of the phosphate is 1.5 μm or less, red rust is generated, and like Comparative Example 5, it can be seen that when the molecular size of the phosphate is 12.5 μm or more, white rust is generated.

That is, like Example 2, it can be seen that when the molecular size of the phosphate is 1.5 to 2.5 μm, the surface state is the best and a unilateral peeling width is also the smallest.

As a result, when the size of the phosphate is 1.5 μm or less, the phosphate is easily dissolved by strong alkali, and when the size of the phosphate is 2.5 μm or more, there is a problem in that a pinhole defect is generated in the coating process. Like the present invention, it can be seen that when the molecular size of the phosphate of the composition for the phosphate film is 1.5 to 2.5 μm, the most preferable property as the phosphate film may be obtained.

Meanwhile, generally, a phosphate film is constituted by hopeite (Zn₃(PO₄)₂4H₂O) and phosphophyllite (Zn₂Fe(PO₄)₂4H₂O) in the related art, but in the phosphate film of the present invention, the hopeite component uses a ternary system of Zn₂Mn(PO₄)₂4H₂O. That is, in the present invention, the composition containing Zn₂Mn(PO₄)₂4H₂O and Zn₂Fe(PO₄)₂4H₂O may be used as the phosphate composition.

In the present invention, the molecular size of the phosphate is limited to 1.5 to 2.5 μm, and the molecular size of the phosphate may be controlled by adjusting an amount of surface modifier providing a nucleation site of the phosphate in a surface modifying process performed before a phosphate film process. That is, when a large amount of surface modifier is added, the size of the phosphate is small, and when a small amount of surface modifier is added, a space where the phosphate is to be sufficiently grown is generated and thus the size of the phosphate is increased.

In the present invention, as the surface modifier, a Zn-based surface modifier may be used, and it is exemplary that the concentration of the surface modifier is maintained to 2.0 to 3.5% in order to control the molecular size of the phosphate to 1.5 to 2.5 μm. Like Comparative Example 4, when the molecular size of the phosphate is 1.5 μm or less, the concentration of the surface modifier may be controlled to be 3.5% or more, and like Comparative Example 5, when the molecular size of the phosphate is 2.5 μm or more, the concentration of the surface modifier may be controlled to be 1.5% or less.

Meanwhile, it is exemplary that the coating weight of the phosphate film coated on the Zn electric-plated steel sheet is 1.6 to 2.0 g/m².

The following Table 3 illustrates a coating weight of the phosphate film according to Comparative Examples and an Example of the present invention.

TABLE 3 Example Coating weight of phosphate film (g/m²) Comparative Example 6 0.94 Example 3 1.63 Comparative Example 7 2.44

The following Table 4 illustrates a dissolved amount of the phosphate film depending on pH according to the coating weight of the phosphate film in Comparative Examples and an Example of the present invention.

TABLE 4 Comparative pH value Example 6 Example 3 Comparative Example 7 pH 8 0.125 0.125 0.125 pH 11 0.25 0 0 pH 13 1.175 1.015 1.375 pH 14 2 1.125 1.875

As illustrated in Tables 3 and 4, when the coating weight of the phosphate film in the Zn electric-plated steel sheet is limited to 1.63 g/m², the dissolved amount at pH 13 is 1.015 g/m² and the dissolved amount at pH 14 is 1.125 g/m². As compared with other Examples in which the coating weight is smaller (Comparative Example 6) or larger (Comparative Example 7) than the coating weight of Example 3, the dissolved amount in Example 3 of the present invention is small in an environment where the alkali degree is high.

That is, in the case of the composition for the phosphate film in the related art, the dissolved amount of phosphate film in the alkali solution is rapidly increased at pH 12 or higher, and by optimizing the coating weight of the phosphate film according to the present invention, the dissolved amount may be significantly reduced even in a strong alkali environment of pH 12 or higher (Table 4). FIGS. 8 and 9 are graphs illustrating solubility according to acidity of a phosphate film in Comparative Examples and an Example of the present invention, and it can be seen that the dissolved amount is reduced in the alkali environment of the present invention.

Meanwhile, the present invention provides a Zn electric-plated steel sheet coated with the composition for the phosphate film. FIG. 10 is a cross-sectional view illustrating a Zn electric-plated steel sheet coated with the composition for the phosphate film according to an exemplary embodiment of the present invention.

The present invention provides a method for coating a phosphate film on the Zn electric-plated steel sheet including steps of forming the phosphate film on the Zn electric-plated steel sheet by treating the composition for the phosphate film and washing the Zn electric-plated steel sheet with the phosphate film.

In the method for coating the phosphate film on the Zn electric-plated steel sheet of the present invention, before forming the phosphate film, i) degreasing, ii) surface-modifying, and iii) washing the Zn electric-plated steel sheet may be further included. The degreasing step means an operation of removing a grease spot and the like on the surface of the steel sheet by using a degreasing solution, and the surface modifying step means a process in which the phosphate film is coated on the surface of the steel sheet well during the phosphate treatment by immersing the steel sheet in a surface modifying solution and generating a nucleation site on the surface of the steel sheet when forming the phosphate film. Further, the washing process means a process of removing the unreacted surface modifying solution coated on the steel sheet.

As such, according to the present invention, even at pH 12 or higher, it is possible to prevent a partial dissolution problem of the phosphate film of the Zn electric-plated steel sheet by obtaining low alkali solubility. That is, generally, for the phosphate film, the dissolved amount is small at pH 11 or lower, but the dissolved amount is rapidly increased at pH 12 or higher. However, according to the present invention, by optimizing the coating weight of the phosphate film, the molecular size of the phosphate, and the Mn content of the phosphate film, even in an alkali environment of pH 12 or higher, the low solubility may be obtained. As a result, the moldability of the Zn electric-plated steel sheet is enhanced.

According to the present invention, the pinhole defect may be significantly reduced according to the fine size of the phosphate, and a beautiful appearance may be obtained by implementing a uniform state of phosphate film and thus marketability is significantly improved.

As described above, the present invention has been described in relation to specific embodiments of the present invention, but the embodiments are only for the purpose of illustration and description. They are not intended to be exhaustive or to limit the invention to the precise forms disclosed, and obviously many modifications and variations are possible in the light of the above teachings. The exemplary embodiments were chosen and described in order to explain certain principles of the invention and their practical application, to thereby enable others skilled in the art to make and utilize various exemplary embodiments of the present invention, as well as various alternatives and modifications thereof. It is intended that the scope of the invention be defined by the Claims appended hereto and their equivalents. 

What is claimed is:
 1. A composition for a phosphate film of a Zn electric-plated steel sheet comprising zinc (Zn), nickel (Ni), and manganese (Mn), wherein a content of Mn is 6 to 8 wt %.
 2. The composition for the phosphate film of the Zn electric-plated steel sheet of claim 1, wherein a molecular size of the phosphate is 1.5 to 2.5 μm.
 3. The composition for the phosphate film of the Zn electric-plated steel sheet of claim 1, wherein a coating weight of the phosphate film coated on the Zn electric-plated steel sheet is 1.6 to 2.0 g/m².
 4. The composition for the phosphate film of the Zn electric-plated steel sheet of claim 1, wherein the composition for the phosphate film comprises Zn₂Mn(PO₄)₂4H₂O and Zn₂Fe(PO₄)₂4H₂O.
 5. A Zn electric-plated steel sheet coated with the composition for the phosphate film of claim
 1. 6. A method for coating a phosphate film on a Zn electric-plated steel sheet, comprising steps of: forming the phosphate film on a Zn electric-plated steel sheet by treating the composition for the phosphate film of claim 1; and washing the Zn electric-plated steel sheet with the phosphate film.
 7. The method for coating the phosphate film on the Zn electric-plated steel sheet of claim 6, further comprising: degreasing, surface-modifying, and washing the Zn electric-plated steel sheet before forming the phosphate film.
 8. The method for coating the phosphate film on the Zn electric-plated steel sheet of claim 7, wherein a surface modifier for modifying a surface of the Zn electric-plated steel sheet is a Zn surface modifier and a concentration thereof is 2.0 to 3.5%. 